1. Field of the Invention
This invention relates to a press for shaping a sheet of metal. More particularly, it relates to a device provided in a press for holding a sheet of metal in position.
2. Description of the Prior Art
It is very difficult to hold the edges of a sheet of metal properly between the upper and lower members of a press. If too strong a force is applied, the sheet is broken, and if the force is too small, it is deformed or curved.
Springs are often used to hold the sheet, but are unsuitable in case a strong elastic holding force is desired as a result of slight displacement, e.g. over a distance of about 1 or 2 mm. There is also known an arrangement employing rubber, but it is also unsuitable, since rubber is too soft.
According to another arrangement known in the art, one of the upper and lower members of a press has a bead surrounding the effective working area of the press, and the bead cooperates with a groove formed in the other press member to hold the metal sheet to be worked. The bead forms the sheet with a concavity extending along its peripheral edges and defining a relatively wide curved portion. This curved portion develops a frictional force which serves to hold the sheet in position. This curved portion has, however, to be removed from a final product. It is necessary to employ a relatively large sheet including an allowance for such a curved portion. A lot of material is wasted. A large space is required for storing not only the sheets to be worked, but also the scrap arising from the removal of the curved edge portions from the final products. These factors have hitherto hindered a reduction in the cost of production and an improvement in productivity.